Chapter 12
Diversity Within Species and
Population Genetics
Mendelian vs. Population Genetics

Mendelian genetics describes the inheritance
and interaction of alleles in individuals.


Example: Each individual has two of the three
possible blood type alleles found in humans
Population Genetics investigates the kinds and
relative abundance of alleles within a population
and changes in alleles over time.
Terminology

A population is a group of organisms that are
potentially capable of breeding naturally and are
found in a specified area at the same time.


All of the beef cattle in Lewis County today.
A species consists of all organisms potentially
capable of breeding naturally among
themselves and having offspring that also
interbreed successfully.

All of the beef cattle in the world today.
Gene Pool

A gene pool consists of all the alleles of all the
individuals of a population.

While any individual may possess only two alleles,
a population may many alleles for the same gene.
Gene Pool


In this “population”, there
are 64 possible alleles
for blood type (32
individuals, each with 2
alleles).
25 copies of Type A


21 copies of the B allele


40.3%
33.9%
16 copies of the O allele

25.8%
Allele Frequency




Allele Frequency is
how often an allele is
found in a population.
The allele frequency
can differ from
population to
population.
We can see an
example with these
mice.
Some populations
have unique alleles.
Allele Frequency

Different physical characteristics are displayed
by people from different parts of the world.

In Europe


In Africa


High allele frequency for light skin, straight hair, and a
narrow nose.
High allele frequency for dark skin, curly hair, and flat
nose
In Asia

Moderately colored skin, straight hair and broad noses
Blood Type Frequency



While the B alleles is the
rarest blood allele (16%),
it is far more common in
Central Asia and very low
in North America among
native populations.
Overall 21% of humans
have an A allele.
It is very common among
Blackfoot Indians,
Australian natives, and
Saami of Northern
Scandinavia.
Subspecies
•
Subspecies are populations of individuals who share
similar phenotypes (and genotypes) which are different
from other populations with different phenotypes (and
genotypes).
•
The northern, Lake Erie, and midland water snakes are
subspecies of Nerodia sepidon.
•
The Lake Erie populations (on islands in the lake) lack
lighter patches found in the mainland northern water
snake populations.
Genetic Diversity
•
Genetic Diversity describes differences among
members of a population.
–
High genetic diversity indicates many different
kind of alleles for each characteristic.
–
Low genetic diversity indicates that nearly all
the individuals in the population have the same
alleles.
–
Mutations
–
Sexual Reproduction
–
Migration
–
Population size
Genetically Distinct Populations
•
There are four reasons for genetically distinct
populations
–
Adaptions to local environmental conditions
–
Founder Effect
–
Genetic Bottleneck
–
Barrier to movement
Genetically Distinct Populations
•
Adaptions to Local Environmental Conditions
–
In local populations, certain adaptions are
valuable for survival in the local environment
•
•
•
In the White Sands National Monument in NM,
has white gypsum sand, so many of the lizards
and mice are light colored
There are blind fish in waters where sunlight
doesn't reach.
Humans have appendixes, that at one time might
have been used for everyday use
Genetically Distinct Populations
•
The founder effect is the concept that small,
newly established populations are likely to
have reduced genetic diversity because of the
small number of individuals in the founding
populations
–
Example: Evolution of organisms on volcanic
islands
–
Galapagos Islands were probably derived from
one or few initial founders
Genetic Bottleneck
•
Genetic Bottleneck is when there is a sharp
reduction in population size due to a chance
event, that results in a reduction in genetic
diversity in subsequent generations.
–
All living cheetahs are practically
identical genetically
–
Overhunting reduced the population of
northern elephant seals to as few as
20 individuals at the end of 19th
century. The population is now over
30,000 but they have far, far less
genetic variation than southern
elephant seals which were not hunted
as severely.
Barriers to Movement
•
With the presence of barriers to free
movement, plants and animals are divided into
small, separate populations
–
Plants and animals that live in lakes are going
to be divided by land
•
This would lead to different allele frequencies
from lake to lake
Genetic Diversity in Domesticated Plants and Animals
•
In order to create more ideal products,
scientists artificially construct specific genetic
combinations that are useful or more
desirable.
– Rapid growth, resistance to disease, etc.
•
Cloning is the process of reproducing
organisms asexually, to create several
genetically identical individuals.
– Plants are cloned to create a larger food supply
Selective Breeding
•
•
In order to preserve
desirable traits from
certain plants or animals,
the mating of individual
organisms are controlled.
–
Intraspecific hybrids
are produced by
controlled breeding
within the same
species.
–
Interspecific hybrids
hybrids between
two species
Beefalo is an
interspecific hybrid
between cattle and
bison.
Genetic Engineering & Monoculture
•
Genetic engineering is to manipulate particular
pieces of DNA and transfer them into a
specific host.
•
Monoculture is a kind of agriculture in which
large areas are exclusively planted with a
single crop with a very specific genetic makeup
–
Makes it easier to manage crops, but are
susceptible to disease due to reduced genetic
diversity
Human Population Genetics
•
Even though human populations are not as
isolated from one another as they were before
easy travel, many people still tend to have
children with those who are similar in race,
economic background, and socially.
–
Non-random mating (at an extreme, inbreeding
over many generations) can elevate the
frequency of otherwise rare alleles.
Tay-Sachs Disease
• Tay-Sachs disease is a homozygous recessive
disease in which some fatty acids are not metabolized
and accumulate in nerve cells in the brain.
This leads to deterioration of mental and physical abilities
until death occurs before age 3.
• One form of the Tay-Sachs allele is especially common
in Ashkenazi Jews, including U.S. Jews who emigrated
from eastern Europe.
Fig. 12.13, pg. 253
Eugenics
• Eugenics are attempts to apply genetic knowledge
directly for the improvement of human existence.
– Greek root meaning, “well-born”
• Positive Eugenics encourage the breeding of positive
characteristics
– Superior Intelligence, Artistic Talent
• Negative Eugenics are the attempt to restrict the
reproduction of parents displaying unfavorable
characteristics
– Low intelligence, mental retardation and criminal behavior
Eugenics in the US
• In the early 1900’s in the US, there was a
eugenics movement to require sterilization of
those considered “genetically inferior”
– Over half the states passed laws requiring the
sterilization of “imbeciles, idiots, convicted rapists,
and habitual criminals”*
– By 1931, the law applied to include, “sexual
perverts, drug fiends, drunkards, and eqileptics”*
– Today, there are still groups and individuals who still
believe in getting rid in “bad genes”
*The sterilization of Carrie Buck by J.D. Smith and K.R. Nelson
Eugenics in WWII
• In Nazi Germany during the 1930’s the concept
of achieving a superior, racially pure group
– Said to have been modeled after the US sterilization
programs
– First included those considered socially and
physically defected
– Then expanded to include entire ethnic groups,
including Jews and Gypsies
– Ended in the mass murder of the Holocaust